Package and printed circuit board attachment

ABSTRACT

Generally, the present disclosure provides example embodiments relating to a package attached to a printed circuit board (PCB). In an embodiment, a structure includes a PCB. The PCB has ball pads arranged in a matrix. Outer ball pads are along one or more outer edges of the matrix, and each of the outer ball pads has a first solder-attach area. Inner ball pads are interior to the matrix, and each of the inner ball pads has a second solder-attach area. The first solder-attach area is larger than the second solder-attach area.

BACKGROUND

In the electronics industry, generally, integrated circuits are formedon semiconductor dies. The features of the integrated circuits on thesemiconductor dies are becoming progressively smaller with advances insemiconductor processing. Semiconductor dies (with integrated circuits)are commonly packaged in packages that contain an interconnect. Theinterconnect of the package can be formed as an integral part of thepackage or can be formed independently of other components of thepackage (such as a package substrate). The interconnect in the packagegenerally provides an interface between the integrated circuit of thesemiconductor die and another component. The interconnect in the packagecan be formed with larger feature sizes, which can be formed with moremature and less expensive technology.

Packages, and possibly other surface mount devices, can then be attachedto a printed circuit board (PCB). The PCB can be a substrate to whichany number of components are attached to form a system-level device, forexample. The PCB can be formed with feature sizes that are even largerthan the feature sizes of the interconnect in the package, and hence,can be formed using even more mature and less expensive technology.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a cross-sectional view of a package mechanically attached andelectrically coupled to a printed circuit board (PCB) in accordance withsome embodiments.

FIG. 2 is a cross-sectional view of a portion of the package substratein accordance with some embodiments.

FIG. 3 is a layout view of a corner portion of the package substrate inaccordance with some embodiments.

FIG. 4 is a cross-sectional view of a portion of the PCB in accordancewith some embodiments.

FIG. 5 is a layout view of a portion of the PCB in accordance with someembodiments.

FIGS. 6A and 6B are cross-sectional views of the package mechanicallyattached and electrically coupled to the PCB in accordance with someembodiments.

FIG. 7 is a flow chart for forming a package attached to a PCB inaccordance with some embodiments.

FIG. 8 is a flow chart for forming a package in accordance with someembodiments.

FIG. 9 is a flow chart for forming a PCB in accordance with someembodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Generally, the present disclosure provides example embodiments relatingto a package attached to a printed circuit board (PCB). The package andPCB are attached by using solder (e.g., solder balls). The PCB has padsto which the solder is to be attached. The pads are arranged in amatrix, and pads along outer one or more rows and/or columns have alarger area to which the solder is to be attached than pads interior tothe matrix. The larger area of the outer pads can reduce a risk ofbridging and short circuiting of solder at the outer rows and/or columns(and more particularly, at corners of the matrix). The risk of bridgingand short circuiting may result from lateral bulging of the solder dueto a reduced height of the solder caused by warpage of the package dueto thermal cycling. Other benefits may be achieved.

Some variations of the example methods and structures are described. Aperson having ordinary skill in the art will readily understand othermodifications that may be made that are contemplated within the scope ofother embodiments. Although method embodiments may be described in aparticular order, various other method embodiments may be performed inany logical order and may include fewer or more steps than what isdescribed herein. In some figures, some reference numbers of componentsor features illustrated therein may be omitted to avoid obscuring othercomponents or features; this is for ease of depicting the figures.

FIG. 1 illustrates a cross-sectional view of a package 20 mechanicallyattached and electrically coupled to a PCB 22 in accordance with someembodiments. The package 20 includes a package substrate 24 and one ormore dies 26 on the package substrate 24. The one or more dies 26 areencapsulated on the package substrate 24 by an encapsulant 28, such as amolding compound.

The package 20 can be any package. As illustrated, the package 20includes a package substrate 24, but such a package substrate can beomitted in other examples, such as when the package has in integratedinterconnect like in an integrated fan-out package. The one or more dies26, which can include any appropriate integrated circuitry, can bemechanically attached and electrically coupled to the package substrate24 by any suitable technique. For example, the one or more dies 26 canbe mechanically attached and electrically coupled to the packagesubstrate 24 using flip chip technology. Controlled collapse chipconnects (C4) can be implemented on the one or more dies 26 and can beused to attach the one or more dies 26 to the package substrate 24. Inanother example, the one or more dies 26 can be mechanically attached tothe package substrate 24 by an adhesive and can be electrically coupledto the package substrate 24 by wire bonding. Any other technology can beused to mechanically attach and electrically couple the one or more dies26 to the package substrate 24. If more than one die 26 is included inthe package 20, any combination of technologies, e.g., flip chip, wirebonding, etc., can be implemented to mechanically attach andelectrically couple the dies 26 to the package substrate 24.

After the one or more dies 26 are mechanically attached and electricallycoupled to the package substrate 24, the one or more dies 26 areencapsulated on the package substrate 24. In some examples, the one ormore dies 26 are encapsulated by a molding compound using compressionmolding, transfer molding, or another molding process.

The package substrate 24 includes a number of metal layers that includevias and lines that route interconnections. The package substrate 24 canredistribute and/or interconnect various signals and/or componentsthrough the vias and/or lines of the metal layers. Additional details ofexample package substrates are described below.

The package 20 is mechanically attached and electrically coupled to thePCB 22 by solder balls 30. The solder balls 30 are or include alead-free solder, such as tin, silver, copper (Sn—Ag—Cu or SAC) solder,or another solder. Solder can be formed on pads of the package substrate24, such as by printing, plating, evaporation, or another process. Thepackage 20 can be placed on the PCB 22 such that the solder aligns withrespective pads on the PCB 22, and a reflow process can be performed toreflow the solder thereby forming the solder balls 30 that attach thepackage 20 to the PCB 22. Multiple packages can be mechanically attachedand electrically coupled to the PCB 22.

The PCB 22 includes multiple metal layers that each includes lines,which may be interconnected between layers by through-hole connectors.The PCB 22 can redistribute and/or interconnect various signals and/orcomponents through the through-hole connectors and/or lines. The PCB 22with various packages attached thereto may implement a system or portionthereof. Additional details of example PCBs are described below.

FIG. 2 illustrates a cross-sectional view of a portion of the packagesubstrate 24. The package substrate 24 includes a core 40. The core 40provides mechanical strength and rigidity for the package substrate 24.The core 40, in some embodiments, is or includes a layer of pre-preg(e.g., a fiberglass matrix injected with an epoxy resin, such as FR-4).The layer of pre-preg can have a metal foil (e.g., copper foil) onopposing sides. Through-holes can be formed through the layer ofpre-preg and plated with a metal (e.g., copper) to form through-holeconnectors 42. The metal foil on the opposing sides can be etched usingphotolithography and etch processes to form metal lines on the opposingsides. Hence, the core 40 can include through-hole connectors 42electrically coupled to various metal lines 44, 46 on opposing sides ofthe core 40.

Various levels of insulating layers and metal layers are then formed onthe core 40. For convenience, “front-side” is used herein to designatethe side of the core 40 on which the one or more dies 26 are to beattached, and “back-side” is used herein to designate the side of thecore 40 opposite from the front-side.

As illustrated, a first front-side insulating layer 50 is formed on thecore 40 and metal lines 44. The first front-side insulating layer 50, insome examples, is an Ajinomoto Build-up Film (ABF) or the like, and islaminated or formed by another process on the core 40 and metal lines44. Via openings are formed through the first front-side insulatinglayer 50 to underlying metal lines 44 using laser drilling, for example.A metal seed layer is formed on the first front-side insulating layer50, such as by using physical vapor deposition (PVD), and a photoresistis formed and patterned over the metal seed layer. A plating process(such as electroless or electroplating) is performed to form metal lines52 and vias (not individually numbered) to connect the metal lines 52with the underlying metal lines 44. The photoresist is then removed,such as by using a wet stripping process, and the exposed metal seedlayer is removed, such as by a wet etching process. The metal seed layercan be or include copper, titanium, another metal, or a combinationthereof, and the metal lines 52 and vias can be or include copper,another metal, or a combination thereof. A second front-side insulatinglayer 54 and die-connection pads 56 with vias are formed on the firstfront-side insulating layer 50 and metal lines 52. The second front-sideinsulating layer 54 and die-connection pads 56 with vias can be formedusing the same processes as described with respect to the firstfront-side insulating layer 50 and metal lines 52 with vias. Thedie-connection pads 56 can be configured and arranged according to howthe one or more dies 26 are to be attached to the package substrate 24.For example, the die-connection pads 56 can be configured and arrangedto accommodate flip chip connections, wire bonding, or otherconnections.

A first back-side insulating layer 60 is formed on the core 40 and metallines 46. Metal lines 62 and vias (not individually numbered) are formedto connect the metal lines 62 with the underlying metal lines 46. Thefirst back-side insulating layer 60 and metal line 62 with vias can beformed using the same processes as described with respect to the firstfront-side insulating layer 50 and metal lines 52 with vias. A secondback-side insulating layer 64 and ball pads 66 with vias are formed onthe first back-side insulating layer 60 and metal lines 62. The secondback-side insulating layer 64 and ball pads 66 with vias can be formedusing the same processes as described with respect to the firstfront-side insulating layer 50 and metal lines 52 with vias. The ballpads 66 can be configured and arranged according to a ball grid array(BGA) matrix, for example, on which the solder balls 30 can be formed.

The one or more dies 26 can be attached to the package substrate 24 atvarious times of forming the package substrate 24. For example, the oneor more dies 26 can be attached (such as by flip chip connections, wirebonding, etc.) on the front-side of the package substrate 24 afterback-side processing (e.g., forming back-side insulating layers 60, 64,metal lines 62, and ball pads 66) is performed. In other examples, theone or more dies 26 can be attached on the front-side of the packagesubstrate 24 before back-side processing is performed. In such examples,the one or more dies 26 can be attached on the front-side of the packagesubstrate 24 and encapsulated by the encapsulant 28 (such as describedabove) before back-side processing is performed.

The package substrate 24 is merely an example. Any number of insulatinglayers and metal layers including metal lines and vias can be formed onthe front-side and/or the back-side of the core. In some examples,package substrates can omit a core and any associated components. Apackage substrate can be formed by any process according to anytechnology.

FIG. 3 illustrates a layout view of a corner portion 24 a of the packagesubstrate 24 in accordance with some embodiments. In some examples, alayout of the package substrate 24 is rectangular, such as a square orrectangle. In such examples, the corner portion 24 a is representativeof each of the four corners in the layout of the package substrate 24.In the layout, ball pads 66 are arranged in a matrix comprising rows (asreferenced herein, x number of rows) and columns (as referenced herein,y number of columns). Although not necessarily illustrated, the matrixcan include ball pads 66 throughout the area of the matrix according tothe rows and columns, or can omit ball pads in some locations, such asin a center area of the layout of the package substrate 24. Asillustrated, the matrix includes rows of ball pads 66, where a first rowincludes ball pads 66-1 j, a second row includes ball pads 66-2 j, athird row includes ball pads 66-3 j, etc. (where j is 1 to y). Thematrix includes columns of ball pads 66, where a first column includesball pads 66-i 1, a second column includes ball pads 66-i 2, a thirdcolumn includes ball pads 66-i 3, etc. (where i is 1 to x). A pitch Pbetween neighboring ball pads 66 along a row or along a column can be ina range from about 200 μm to about 1000 μm, such as about 1000 μm. Asillustrated in FIG. 3, the pitch P is between ball pads 66-51 and 66-61in a column, and is between ball pads 66-61 and 66-62 in a row. The ballpads 66 have a first diameter D1, and have a first spacing S1 betweenneighboring ball pads 66 along a row or along a column. The firstdiameter D1 plus the first spacing S1 is equal to the pitch P. The firstdiameter D1 can be in a range from about 100 μm to about 600 μm, such asabout 500 μm, and the first spacing S1 can be in a range from about 100μm to about 500 μm, such as about 500 μm. Although described as having adiameter and being illustrated as circular, the ball pads 66 can haveany geometry, such as any polygon.

In the examples where the layout of the package substrate 24 isrectangular, the outer rows of the matrix of ball pads 66 (e.g., ballpads 66-1 j and 66-xj) and outer columns of the matrix of ball pads 66(e.g., ball pads 66-i 1 and 66-iy) also form a rectangular shape. Hence,the matrix of ball pads 66 includes four corner ball pads 66-11, 66-1 y,66-x 1, and 66-xy. Each of the respective four corner ball pads 66-11,66-1 y, 66-x 1, and 66-xy is the respective ball pad 66 closest to acorner of the layout of the package substrate 24. For example, asillustrated, the corner ball pad 66-11 is the ball pad 66 of the matrixclosest to the corner 24 c of the layout of the package substrate 24.

FIG. 3 further illustrates a cross-section 6B that is illustrated infurther detail in FIG. 6B. The cross-section 6B is across ball pads66-11, 66-22, 66-33, and 66-44.

FIG. 4 illustrates a cross-sectional view of a portion of the PCB 22 inaccordance with some embodiments. The PCB 22 includes a number ofinsulating layers 80, 84, 88, 92, 96, 100, 104, 108, 112, 116, 120, and124 and a number of metal layers. The various metal layers asillustrated include metal lines 82, 86, 90, 94, 98, 102, 106, 110, 114,118, and 122. In some examples, the PCB 22 includes ten to thirtydifferent insulating layers with a metal layer disposed between eachneighboring pair of insulating layers.

In some examples, each of the insulating layers 80, 84, 88, 92, 96, 100,104, 108, 112, 116, 120, and 124 of the PCB 22 is or includes a layer ofpre-preg (e.g., a fiberglass matrix injected with an epoxy resin, suchas FR-4). The layer of pre-preg can initially have a metal foil (e.g.,copper foil) on one or both opposing sides. Each metal foil is patternedinto the corresponding metal lines 82, 86, 90, 94, 98, 102, 106, 110,114, 118, and 122 using photolithography and etch processes. After themetal foils are patterned to form corresponding metal lines, theinsulating layers are joined together. The insulating layers are alignedand pressed together to bond the insulating layers.

After the insulating layers are joined, through-hole connectors 126 areformed through the insulating layers. Holes can be formed through thejoined insulating layers using drilling, for example. After forming theholes, the joined insulating layers may be plated with a metal (e.g.,copper and/or tin). The plating forms the through-hole connectors 126 inthe holes and can also form a metal layer on the exterior surfaces ofthe joined insulating layers. The metal layers on the exterior surfacesare patterned. On an exterior surface, ball pads 128 with metal linesconnecting the ball pads 128 to the through-hole connectors 126 arepatterned. On another exterior surface, metal lines 130 or otherpatterns are patterned. The patterning of the ball pads 128 with metallines and/or the metal lines 130 on the exterior surfaces can beimplemented by photolithography and etching processes, and/or aphotolithography process and the plating that deposits the metal.

Solder masks (or solder resists) 132 and 134 are formed on respectiveexterior surfaces of the joined insulating layers. The solder masks 132and 134 can be patterned to expose an underlying metal pattern. Forexample, as illustrated, the solder mask 132 is patterned, using aphotolithography process, to define openings 136 that expose the ballpads 128.

FIG. 5 illustrates a layout view of a portion 22 a of the PCB 22 inaccordance with some embodiments. In the layout, ball pads 128 arearranged in a matrix comprising rows (as referenced herein, x number ofrows) and columns (as referenced herein, y number of columns). Thematrix of ball pads 128 corresponds to the matrix of ball pads 66 on thepackage substrate 24. Although not necessarily illustrated, the matrixcan include ball pads 128 throughout the area of the matrix according tothe rows and columns, or can omit ball pads in some locations, such asin a center area of the layout of the matrix. As illustrated, the matrixincludes rows of ball pads 128, where a first row includes ball pads128-1 j, a second row includes ball pads 128-2 j, a third row includesball pads 128-3 j, etc. (where j is 1 to y). The matrix includes columnsof ball pads 128, where a first column includes ball pads 128-i 1, asecond column includes ball pads 128-i 2, a third column includes ballpads 128-i 3, etc. (where i is 1 to x). Each ball pad 128 is exposedthrough a respective opening 136 through the solder mask 132. The pitchP is between neighboring ball pads 128 and/or openings 136 along a rowor along a column.

Ball pads 128 along outer one or more rows and columns have a largerdiameter than ball pads 128 interior to the matrix of ball pads 128. Asa result, the ball pads 128 along the outer rows and columns have alarger area than ball pads 128 interior to the matrix of ball pads 128.In some examples, each of the ball pads 128 along the outer one or morerows and columns has an area that is in a range from about 10% to 50%larger than an area of each ball pad 128 interior to the matrix. Asillustrated in FIG. 5, the outer three rows (e.g., ball pads 128-1 j,128-2 j, 128-3 j, 128-(x−2)j, 128-(x−1)j, and 128-xj) and outer threecolumns (e.g., ball pads 128-i 1, 128-i 2, 128-i 3, 128-i(y−2),128-i(y−1), and 128-iy) along the periphery of the matrix have thelarger diameter and area. In other examples, any number of outer rowsand outer columns of ball pads 128 along each edge of the matrix may beimplemented to have a larger diameter and area than interior ball pads128. For example, in some embodiments, one to five outer rows andcolumns along each edge of the matrix may be implemented to have alarger diameter and area than interior ball pads 128. Althoughillustrated and described as the number of rows equaling the number ofcolumns, in other examples, the number of outer rows having the largerdiameter and area may differ from the number of outer columns having thelarger diameter and area.

The larger ball pads 128 (e.g., ball pads 128-1 j,128-i 1, etc.) have asecond diameter D2, and the smaller ball pads 128 (e.g., ball pads128-64, etc.) have a third diameter D3. Each of the openings 136 has afourth diameter D4. The fourth diameter D4 is greater than each of thesecond diameter D2 and the third diameter D3. In some examples, thesecond diameter D2 is in a range from about 110 μm to about 600 μm, suchas about 600 μm; the third diameter D3 is in a range from about 100 μmto about 550 μm, such as about 525 μm; and the fourth diameter D4 is ina range from about 130 μm to about 650 μm, such as about 650 μm.Although described as having a diameter and being illustrated ascircular, the ball pads 128 can have any geometry, such as any polygon.

First gaps are defined between the edges of larger ball pads 128 andrespective sidewalls of the openings 136. The first gaps have a secondspacing S2 between the edge of the ball pad 128 and the sidewall of theopening 136. Second gaps are defined between the edges of smaller ballpads 128 and respective sidewalls of the openings 136. The second gapshave a third spacing S3 between the edge of the ball pad 128 and thesidewall of the opening 136. The third spacing S3 is larger than thesecond spacing S2. In some examples, the second spacing S2 is in a rangefrom about 15 μm to about 30 μm, such as about 25 μm, and the thirdspacing S3 is in a range from about 20 μm to about 50 μm, such as about50 μm. In other examples, the openings 136 can have varying diameters,and/or the spacing of gaps formed in the openings 136 can be equal orvary throughout the matrix. Although described as having a diameter andbeing illustrated as circular, the openings 136 can have any geometry,such as any polygon.

As illustrated and described the ball pads 128 are non-solder maskdefined pads. The openings 136, as illustrated, are larger than theareas of the ball pads 128, which forms the first and second gapsbetween the edges of the ball pads 128 and the sidewalls of the opening136. In other examples, the ball pads may be solder mask defined pads.In such examples, the ball pads throughout the matrix may or may nothave a same diameter and area. In such examples, openings through thesolder mask 132 may differ in diameter such that areas of ball padsexposed through the opening to which solder balls may be formed alsodiffer in diameter. For example, openings 136 along outer rows andcolumns of the matrix can have a larger diameter than openings 136interior to the matrix. The openings 136 define the areas of the ballpads on which the solder balls are to be formed, and hence, exposedareas of ball pads along the outer rows and columns are larger thanexposed areas of ball pads interior to the matrix.

FIG. 5 further illustrates a cross-section 6B that is illustrated infurther detail in FIG. 6B. The cross-section 6B is across ball pads128-11, 128-22, 128-33, and 128-44.

FIGS. 6A and 6B illustrate cross-sectional views of the package 20mechanically attached and electrically coupled to the PCB 22 inaccordance with some embodiments. FIG. 6B illustrates a corner portion140 shown in FIG. 6A. FIGS. 6A and 6B illustrate the package 20 and PCB22 after thermal cycling, such as after a reflow process to reflow thesolder balls 30. In some examples, a coefficient of thermal expansion(CTE) of the encapsulant 28 (e.g., molding compound) and/or othercomponents on the package substrate 24 is greater than a CTE of thepackage substrate 24. Hence, when the package 20 is heated during areflow process, the encapsulant 28 and/or other components can expand agreater amount than the package substrate 24. The difference in theamount of expansion can cause warpage of the package 20. As illustrated,when the encapsulant 28 and/or other components expand a greater amountthan the package substrate 24, a bottom surface of the package substrate24 (e.g., to which the solder balls 30 are attached) can become concave.Locations in the package 20 farthest from a center of the package 20 canexperience a greatest warpage deflection WD. For example, when a layoutof the package 20 is rectangular, such as described above, corners ofthe layout are generally the locations in the package 20 farthest fromthe center of the package 20, and hence, a greatest warpage deflectionWD can be observed at the corners. Even further, the larger the layoutof the package 20 is (such as for a 50 mm×50 mm layout), the larger thewarpage deflection WD can become.

When the warpage deflection WD occurs during reflowing of the solderballs 30, a volume available to outer solder balls, and moreparticularly, solder balls 30 at the corners, to flow can be decreased.The warpage deflection WD can cause a height between ball pads 66 on thepackage substrate 24 and ball pads 128 on the PCB 22 to decrease.Assuming that the ball pads 66 on the package substrate 24 have a sameattachment area and pitch and that the ball pads 128 on the PCB 22 havea same attachment area and pitch, solder balls 30 at the corners risklaterally bulging to accommodate the reduced height due to warpage. Thisbulging can cause bridging and short circuiting between solder balls 30.

According to some embodiments, the attachment area of the ball pads 128is larger along outer rows and columns in the layout of the matrix ofball pads 128 on the PCB 22. The larger attachment area can provide alarger wetting surface for the solder balls 30 attached to those ballpads 128. The larger wetting surface on the ball pads 128 along theouter rows and columns permits an accommodation in volume to offset areduced height due to warpage of the package 20. The accommodation involume can reduce lateral bulge of the solder balls 30 along the outerrows and columns, which can reduce a risk of bridging and shortcircuiting.

FIG. 6B illustrates some components illustrated in the layout views ofFIGS. 3 and 5. More specifically, FIG. 6B illustrates ball pads 66-11,66-22, 66-33, and 66-44 on the package substrate 24 shown in FIG. 3 andball pads 128-11, 128-22, 128-33, and 128-44 on the PCB 22 shown in FIG.5. Corresponding solder balls 30-11, 30-22, 30-33, and 30-44 areattached to the ball pads 66-11, 66-22, 66-33, and 66-44 and the ballpads 128-11, 128-22, 128-33, and 128-44. The ball pads 66-11, 66-22, and66-33, and ball pads 128-11, 128-22, and 128-33 are in the three outerrows/columns of the matrix as illustrated in FIGS. 3 and 5, and the ballpad 66-44 and ball pad 128-44 are interior to the three outerrows/columns as illustrated in FIGS. 3 and 5. As previously described,the ball pads 128-11, 128-22, and 128-33 have the second diameter D2,while the ball pad 128-44 has the third diameter D3, which is less thanthe second diameter D2. The second diameter D2 permits the ball pads128-11, 128-22, and 128-33 to have a greater area, and hence, a greaterwetting surface, than the ball pad 128-44 with the third diameter D3.

FIG. 6B further illustrates a first height H1 of the solder ball 30-11in an outer column of the matrix and a second height H2 of the solderball 30-44 in an interior of the matrix. The first height H1 is lessthan the second height H2. The difference between the first height H1and the second height H2 can be the result of warpage. As illustrated,the larger second diameter D2 of the ball pad 128-11 can accommodate thereduced first height H1 of the solder ball 30-11 relative to the smallerthird diameter D3 of the ball pad 128-44 and the second height H2 of thesolder ball 30-44. Hence, lateral bulge and short circuiting of solderballs 30-11, 30-22, and 30-33 can be reduced.

FIG. 7 is a flow chart for forming a package attached to a PCB inaccordance with some embodiments. In operation 202, a package is formed,such as to be described subsequently with reference to FIG. 8. Inoperation 204, a PCB is formed, such as to be described subsequentlywith reference to FIG. 9. In operation 206, the package is attached tothe PCB. For example, solder can be formed on ball pads on the packageand/or the PCB. The package can be placed on the PCB and aligned suchthat the ball pads on the package aligns with appropriate ball pads onthe PCB with the solder disposed therebetween. The solder is thenreflowed to form a more permanent mechanical and electrical attachmentbetween the ball pads of the package and the ball pads of the PCB.

FIG. 8 is a flow chart for forming a package in accordance with someembodiments. In operation 222, a core is formed with through-holeconnectors. The core can be formed as described above with respect toFIG. 2. In operation 224, front-side processing is performed to form oneor more insulating layers with one or more metal layers on thefront-side of the core. Any number of insulating layers and metal layersmay be formed on the front-side as described above with respect to FIG.2. In operation 226, back-side processing is performed to form one ormore insulating layers with one or more metal layers on the back-side ofthe core. Any number of insulating layers and metal layers may be formedon the back-side as described above with respect to FIG. 2. Theinsulating layers on the front-side and back-side, the metal layersdisposed in the insulating layers, and the core form a packagesubstrate. In operation 228, one or more dies are attached to thefront-side of the package substrate, such as described above withrespect to FIG. 2. In operation 230, the one or more dies areencapsulated on the front-side of the package substrate. For example, anencapsulant, such as a molding compound, may be used to encapsulate theone or more dies, such as by using compression molding, transfermolding, or another molding process.

FIG. 9 is a flow chart for forming a PCB in accordance with someembodiments. In operation 242, metal layers are formed on insulatinglayers. The metal layers (e.g., with patterned metal lines) can beformed on the insulating layers as described above with respect to FIG.4. In operation 244, the insulating layers with the metal layers can bejoined together, such as described above with respect to FIG. 4. Inoperation 246, through-hole connectors are formed through the joinedinsulating layers, such as described above with respect to FIG. 4. Theforming of the through-hole connectors may further form metal layers onexterior surfaces of the joined insulating layers. In operation 248,solder masks are formed on the exterior surfaces and are patterned toexpose ball pads.

Some embodiments can achieve advantages. For example, as describedabove, a risk of bridging and short circuiting of solder balls in outerrows and columns of a matrix can be reduced, which can increase yield ofpackages attached to PCBs. Further, in other configurations, no solderballs were formed proximate corner portions of a layout of a package(e.g., depopulation in the corner portions) due to the associated riskof bridging and short circuiting. In some embodiments, solder balls maybe formed in corner portions of a layout of a package, which canincrease the number of inputs and/or outputs from the package.

An embodiment is a structure. The structure includes a printed circuitboard (PCB). The PCB has ball pads arranged in a matrix. Outer ball padsare along one or more outer edges of the matrix, and each of the outerball pads has a first solder-attach area. Inner ball pads are interiorto the matrix, and each of the inner ball pads has a secondsolder-attach area. The first solder-attach area is larger than thesecond solder-attach area.

Another embodiment is a structure. The structure includes a package, aprinted circuit board (PCB), and solder balls. The package has firstsolder ball pads arranged in a first matrix. A layout of the package isa first rectangular shape. The first matrix of the first solder ballpads has outer edges in a second rectangular shape. Each corner of thesecond rectangular shape of the first matrix of the first solder ballpads is disposed proximate a respective corner of the first rectangularshape of the layout of the package. The PCB has second solder ball padsarranged in a second matrix corresponding to the first matrix. Arespective corner solder ball pad is at each corner of the second matrixof the second solder ball pads. An interior solder ball pad is interiorto the second matrix of the second solder ball pads. The solder ballsare mechanically attached to the first solder ball pads and the secondsolder ball pads. Each corner solder ball pad has a first area to whicha respective corner one of the solder balls is attached. The interiorsolder ball pad has a second area to which an interior one of the solderballs is attached. The first area is greater than the second area.

A further embodiment is a method. A package is attached to a printedcircuit board (PCB). The attaching includes reflowing solder balls.After reflowing the solder balls, each of the solder balls is attachedto a respective one of first pads on the package and to a respective oneof second pads on the PCB. The second pads are arranged in a matrix onthe PCB. A first one of the second pads is disposed at a corner of thematrix, and a second one of the second pads is disposed interior in thematrix. The first one of the second pads has a first area to which arespective one of the solder balls is attached, and the second one ofthe second pads has a second area to which a respective one of thesolder balls is attached. The first area is greater than the secondarea.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A structure comprising: a printed circuit board(PCB) having ball pads arranged in a matrix, outer ball pads being alongone or more outer edges of the matrix, each of the outer ball padshaving a first solder-attach area, inner ball pads being interior to thematrix, each of the inner ball pads having a second solder-attach area,the first solder-attach area being larger than the second solder-attacharea; and a solder mask having openings therethough, wherein each of theopenings that exposes a respective one of the outer ball pads has afirst spacing between an edge of the respective one of the outer ballpads and a sidewall of the solder mask that defines the respectiveopening, and each of the openings that exposes a respective one of theinner ball pads has a second spacing between an edge of the respectiveone of the inner ball pads and a sidewall of the solder mask thatdefines the respective opening; and the second spacing is larger thanthe first spacing.
 2. The structure of claim 1, wherein the outer edgesof the matrix form a rectangular shape.
 3. The structure of claim 1,wherein the outer ball pads are within a number of outer rows and outercolumns, each of the outer rows and the outer columns being parallel toa corresponding one of the outer edges of the matrix, the number beingin a range from one to five.
 4. The structure of claim 1, wherein thefirst solder-attach area is in a range from 10% to 50% larger than thesecond solder-attach area.
 5. The structure of claim 1, wherein theopenings that expose the inner ball pads and the outer ball pads eachhave a same lateral opening dimension.
 6. The structure of claim 1,wherein the first solder-attach area of each of the respective outerball pads and the second solder-attach area of each of the inner ballpads are not defined by the solder mask.
 7. The structure of claim 1further comprising: a package; and solder balls, each of the solderballs being attached to a respective one of (i) the first solder-attacharea of one of the outer ball pads and (ii) the second solder-attacharea of one of the inner ball pads, each of the solder balls furtherbeing attached to the package.
 8. A structure comprising: a packagehaving a first plurality of solder ball pads, each solder ball pad ofthe first plurality being of a same diameter; a printed circuit board(PCB) having a second plurality of solder ball pads, each solder ballpad of the second plurality corresponding to a solder ball pad of thefirst plurality, the second plurality including corner solder ball padsadjacent respective corners of the PCB, and including interior solderball pads in the interior of the second plurality, the respective cornersolder ball pads being larger than the respective interior solder ballpads; solder balls mechanically attached to the first plurality ofsolder ball pads and the second plurality of solder ball pads, eachcorner solder ball pad having a first area to which a respective one ofthe solder balls is attached, each interior solder ball pad having asecond area to which one of the solder balls is attached, the first areabeing greater than the second area, wherein all of the solder balls hasa same material composition and same melting temperature; and a soldermask having openings respectively exposing the solder ball pads, whereinrespective first gaps between the respective openings and the respectiveinterior solder ball pads are greater than respective second gap betweenthe respective openings and the respective corner solder ball pads. 9.The structure of claim 8, wherein the first area is in a range from 10%to 50% larger than the second area.
 10. The structure of claim 8,wherein a height of each solder ball attached to a corner solder ballpad is greater than a height of each solder ball attached to an interiorsolder ball pad.
 11. The structure of claim 8, wherein the corner solderball pads are arranged in rows and columns running parallel torespective outer edges of the second plurality.
 12. The structure ofclaim 8, wherein the PCB includes a solder mask, openings being throughthe solder mask, each of the second solder ball pads being exposed by arespective one of the openings.
 13. The structure of claim 12, whereinthe openings that expose the second solder ball pads each have a samelateral opening dimension.
 14. The structure of claim 12, wherein thefirst area of the respective corner solder ball pads and the second areaof the respective interior solder ball pads are not defined by thesolder mask.
 15. A structure comprising: a package electronically bondedto a printed circuit board (PCB) by way of solder balls, wherein each ofthe solder balls is attached to a respective one of first pads on thepackage and to a respective one of second pads on the PCB, the secondpads being arranged in a matrix on the PCB, the second pads including acorner pad disposed at a corner of the matrix, and an interior paddisposed interior in the matrix, the corner pad having a first area towhich a respective one of the solder balls is attached and being solderconnected to one of the first pads, the interior pad having a secondarea to which a respective one of the solder balls is attached and beingsolder connected to a different one of the first pads, the first areabeing greater than the second area, the first pads being arranged in amatrix on the package, all of the first pads on the package having asame third area.
 16. The structure of claim 15, wherein the first areais in a range from 10% to 50% greater than the second area.
 17. Thestructure of claim 15, wherein the second pads in one to five outer rowsof the matrix and the second pads in one to five outer columns of thematrix each have the first area to which respective ones of the solderballs are attached.
 18. The structure of claim 15, wherein the solderconnecting the corner pad to one of the first pads is a same materialcomposition and same melting temperature as the solder connecting theinterior pad to a different one of the first pads.
 19. The structure ofclaim 15, further comprising: a solder mask on the PCB and including: afirst opening that exposes the first one of the second pads with a firstspacing between an edge of the first one of the second pads and asidewall of the opening, and a second opening that exposes the secondone of the second pads with a second spacing between an edge of thesecond one of the second pads and a sidewall of opening, wherein thesecond spacing is larger than the first spacing.